1. Field of the Invention
The present disclosure relates to a field-effect transistor (FET) including a group-III nitride semiconductor, and a method for producing the FET.
2. Description of the Related Art
Group-III nitride semiconductors (InxAlyGa1-(x-y)N (0≤x≤1, 0≤y≤1) hereinafter also referred to as InAlGaN) represented by gallium nitride (GaN) have a larger band gap (for example, GaN has 3.4 eV at a room temperature), a higher breakdown voltage, and a higher saturation voltage. Thus, the group-III nitride semiconductors have been receiving attention as materials for radio frequency devices or high power switching devices. For example, a heterojunction structure (hereinafter referred to as an AlGaN/GaN heterostructure) of a laminate in which an AlGaN layer is laminated above a GaN layer results in a high polarization field on the (0001) plane of the GaN layer. Thereby, electrons are highly accumulated near the heterojunction in the GaN layer without adding impurities to the GaN layer, that is, a two dimensional electron gas (2DEG) is formed.
GaN materials have a higher saturated drift velocity, for example, a drift velocity two or more times higher than those of GaAs materials that are currently widespread as materials for high-frequency transistors in a high electric field region of approximately 1×105 V/cm. Thus, FETs of the AlGaN/GaN heterostructure can achieve a higher current density.
Instead of AlGaN layers in the AlGaN/GaN heterostructure, an InAlN/GaN heterostructure including an InAlN layer has also been proposed to achieve a higher current density. InxAlyN lattice-matches GaN when x=0.17, that is, x:y=0.17:0.83. Here, InxAlyN has a band gap of approximately 4.7 eV that is larger than that of AlzGa(1-x)N (3.6 eV to 4.3 eV, where z ranges from 0.1 to 0.4) that is generally used. Thus, FETs of the InAlN/GaN heterostructure including the InAlN layer instead of the AlGaN layer have a higher conduction band discontinuity (ΔEc) at the heterojunction between the InAlN and GaN layers, and thus electrons are highly confined. Furthermore, InAlN has spontaneous polarization higher than that of AlGaN. According to these features, the FETs of the InAlN/GaN heterostructure can form a 2DEG in extremely high concentration, and achieve a higher current density.
On the other hand, one of the problems of transistors containing GaN materials is higher contact resistance. Particularly, InAlN surfaces tend to have higher contact resistance due to a larger band gap of InAlN as described above. Thus, some propose techniques for reducing the contact resistance (for example, see Japanese Unexamined Patent Application Publication No. 2007-165431 (hereinafter referred to as Patent Literature (PTL) 1).
As illustrated in FIG. 9, FET 1000 of PTL 1 includes in this order from the bottom to the top: substrate 1111; electron transport layer 1112; barrier layer 1113 containing InAlN; cap layer 1114 containing at least one of InGaN, InN, and GaN; and source electrode 1115 and drain electrode 1116 that are ohmic electrodes. Furthermore, gate electrode 1117 is formed above cap layer 1114. This structure enables FET 1000 to achieve a desired ohmic contact, and reduces the contact resistance.